Paramyxoviruses are the leading cause of childhood respiratory-tract diseases worldwide. In-depth understanding of replication strategies of paramyxoviruses is essential to devise means to eradicate the mortality and morbidity caused by these viruses. The goal of this project is to precisely define functions and structures of the proteins encoded in the polycistronic P/C mRNA of the prototype paramyxovirus, Sendai virus. The P/C mRNA of Sendai virus codes for the highly phosphorylated protein P and four "nonstructural" proteins designated as the C proteins. While the P protein is putatively involved in the RNA-dependent RNA polymerase activity, no functions are known for the C proteins. To define the functions of these viral proteins, systems will be developed to overexpress or abolish the expression of these proteins at various times during the Sendai virus growth cycle. The changes caused by over- or under-expression of these proteins on Sendai virus replication will be monitored to decipher the role of the P and C proteins. Various site-directed mutants of the P/C mRNA will be used to relate structure with function. A complementary in vitro transcription and replication system will be developed to assay function of the purified viral polymerase proteins and their site-directed mutants. Because the P protein is highly phosphorylated, the role and mechanism of P phosphorylation will be examined. It will be important to determine whether P is an autophosphorylating protein and whether its phosphorylation sites are modulated during Sendai virus replication. If the P protein is a protein kinase which is capable of phosphorylating other viral and/or host proteins, it might have a more global role in the virus growth in addition to being the polymerase associated protein. Since P occurs as a disulfide-linked trimer, the role of its trimeric structure will be dissected using site-specific mutants of the P protein lacking one or more cysteine residues. A detailed knowledge of functions and structures of the P and C proteins may provide us clues by which diseases as diverse as measles, mumps, and pneumonia could be controlled.